The invention relates to the field of marine seismic exploration. In the past, marine seismic exploration has been performed for the most part with the use of impulsive sources. For a number of reasons, however, interest in marine vibrators has increased. Unlike an impulsive source, a vibrator can be adjusted easily so that, spectrally, its output energy is tailored to the requirements of a specific exploration objective. Because of the correlation step in processing, vibrator data are less sensitive than are data acquired with impulsive sources to many kinds of noise bursts, including shots from other crews, sounds from shipping lanes and drilling rigs, and activities of marine animals. Two vibrating sources can be run simultaneously with orthogonal sweep functions and data from two adjacent seismic lines acquired simultaneously with a single receiver cable, without having to compromise shotpoint spacing by alternately firing two sources. Since the output of a vibrator is spread over a relatively long time interval, the peak output level of the source is much lower than that of an impulsive source having comparable strength, which reduces possible impact on the environment.
In the normal mode of marine operations, the seismic ship, the source, and the receivers move continuously through the water as data are recorded. Because of that motion, the possibility exists of Doppler shifting of the vibratory signals emitted by the source and detected by the receivers. Methods for correcting the effect of Doppler shifting of the seismic data, however, have not heretofore been known.